Stats speak for themselves. This deck took me from Rank 5 to rank 1 on the first day of the expansion. In total I have played 100 games with the deck, with only 1 card substitution along the way. It does very well against Druid and Priest and only has a losing record against warrior and rogue. I had an 11 game winning streak from Rank 2 - Mid Legend ranks. Mulligans updated below. I will do a more in depth strategy guide at 200 likes.
In all matchups, keep Prince Keleseth, Bloodsail Corsair, and Flame Imp if drawn. This deck wins 72% of the time when Keleseth is in the opening hand. The 2nd best card to keep is Bloodsail Corsair, 3rd is tie between Flame imp and Mortal Coil. Darkshire councilman can be kept in non-agro matchups.
Warrior - Always assume its pirate. Mulligan hard for Flame Imp, Blooodsail Corsair. Go for three 1-drops to match the warrior's agro and Tar Creeper.
Warlock - Keep Flame imp and Bloodsail, otherwise mulligan for Keleseth. Darkshire Councilman can be kept as well.
Shaman - Assume it is token/evolve. Mulligan for flame imp and mortal coil.
Rogue - Assume miracle. Mulligan for 1-drops.
Priest - Mulligan for Keleseth hard. Be careful not to play into Potion of Madness. After turn 5, favor tapping over playing 1-drops. Get Scalebane and Dreadlord onto the board when drawn.
Paladin - Assume it is murloc. Keep multiple 1-drops, control the board.
Mage - Keep 1-drops. Watch out for doomsayer combo turn 5+.
Hunter - Keep 1-drops and mortal coil. You need to win the board early.
Druid - If you suspect agro, keep 1-drops and tar creeper. If you suspect ramp, keep Councilman and 1-drops.
Possessed Villager can be subbed out for a 2nd Sea Giant, Bonemare, or Mortal Coil. The 2nd weakest card is Fire Fly which can be replaced with any of the above 3 cards. I have not tried The Black Knight, but he would improve odds in some matchups. Doomguard has not worked well for me, but may be a good addition if you can live with seemingly always discarding Gul'dan.
Deck stats and August legend rank ---> http://imgur.com/a/ZvxnG